Read about the development of the empty atom concept and the Bohr atom. [2 moderate web pages.] Don't focus on the mathematics, but try to understand the key relationships involved: what physical entities are being described? For example, 1/2mv² is kinetic energy.

• Rutherford discovered that radioactivity decreases geometrically as time passes. How is a geometric progression different from an arithmetic progression?
• How do we explain the very small deflection most alpha particles experience passing near the nucleus?
• Why does direct deflection back from the foil disprove the "plum pudding" model?
• What is a hyperbolic path? How does it differ from a parabolic path or a circular path?
• Why was large angle scattering important in determining the size of the nucleus?
• What did Rutherford originally think the α particle was made of?
• Can an electron have an orbit of any radius when it is part of an atom? Why is this a problem for classical mechanics?
• How is the kinetic energy of an electron related to frequency? What does this frequency measure?
• What did Richard Whiddington's experiments with x-rays show about atoms?
• What is Balmer's formula? What does it predict? What is "n" in the formula?
• How are the radius and speed of the orbit of an electron proportional to "n"?

A more graphic treatment of the different parts of the atom is presented at the Trium Safety Group site, by Noel Griffin.
[7 really short web pages with graphics: read them all:

• Atomic structure
• The Nucleus and Electrons
• Atomic Number
• Atomic Mass
• Electron Orbits
• The size of the electron
• Chemical symbols

• Why is a fixed structure hard to describe for the atom?
• How many types of atoms occur "naturally"? How are "unnatural" atoms produced?
• Why is the mass of the electron usually ignored when determining the mass of an atom?
• What is an electron shell?
• What parts of the atom are affected by chemical reactions (such as the recombination of sugar molecules into carbon dioxide and water)?
• Roughly how much larger than the nucleus is the atom with its electron shell?